1. Technical Field
This invention relates, in general, to "silk screen" printing and more particularly to an improved process and new apparatus for manufacturing and printing with a stencil which provides greater accuracy and significantly reduced distortion.
2. Background Information
"Silk screen" printing is an old and well established art which employs a screen supported stencil to provide a dense and opaque layer of ink on a substrate. The name "silk screen" comes from the threads originally used to support the various individual elements of the stencil. Today, this printing process enjoys widespread commercial application and is used to print on such diverse items as dishes, dials, poster board, plexiglass sheets, textiles and silicon wafers. In the electronics industry the process is sometimes referred to as thick-film printing.
The process involves two phases: stencil manufacture and stencil printing. In the manufacture of a stencil, the screen fabric (now generally polyester, not silk) is stretched tightly across a stable frame to which it is adhered. This stretched screen is then coated with a photosensitive emulsion. A film positive, containing an opaque image on a clear film base of the art to be printed, is placed in contact with the bottom of the emulsion coated screen. Both are then placed in a vacuum frame consisting of a glass plate and a rubber blanket. Air is withdrawn from between the glass and the rubber sandwiching the screen and contiguous positive between the two. U.S. Pat. No. 3,463,587 illustrates such a vacuum frame used for preparing silk screen stencils.
The emulsion coated screen is photo exposed by beaming a strong light through the glass of the vacuum frame to harden all emulsion not masked from the light by the opaque lines of the film positive. The exposed screen is then removed from the vacuum frame to be dampened by a spray of water that dissolves all unhardened areas completing the stencil making process.
The stencil is now ready for printing. This generally requires, in addition to the ink and the substrate to be printed on, three basic appliances. The first is a table to support the stencil and the substrate to be printed on. The table is commonly perforated and attached to a vacuum to hold the substrate securely during the printing process. The second appliance is a hinge/clamp device that attaches the stencil frame to the table so that it may be raised and lowered to the exact same position each time a new substrate is placed on the table for printing. The third commonly used appliance is the squeegee, a resilient scraping device that spreads the ink across the back of the stencil and applies the pressure that causes the ink to pass through the stencil onto the substrate.
With these basic elements, a number of complex mechanisms have been constructed. These include the hand-operated, semi-automatic, three-quarter automatic, fully automatic, as well as cylinder and rotary screen printers. With the exception of the highly specialized rotaries, these prior printers all generally utilize the frame stretched stencil and the squeegee. The cylinder press has a drum bed (table) instead of a flat bed and its stencil reciprocates in a motion with the drum bed instead of hinging or rising to allow the replacement of a substrate.
In general, the screen printing process employs the following sequence of steps: after a register is determined and all adjustments have been made (called set-up) an amount of ink is placed on the stencil outside of the image area, as wide as the image, and a squeegee slightly wider than the image is placed behind the ink, pressed down and moved with even pressure across the image area forcing the ink through the open stencil spaces and onto the substrate. The screen is then lifted and the ink is pushed back over the image area with little pressure returning it to the point of origin and "flooding" the screen in the process. While the screen is raised, the substrate is released and removed for drying and another substrate is placed in registry on the table so the sequence can begin again.
With the exception of rotary screen printers, the sequence described above is generally used by all mechanisms that use a screen/stencil for printing. Variations may occur as system options such as flood bars that return the ink by lifting and carrying it instead of coating the image area; or, as in the case of the cylinder press, a stationary squeegee may be made to traverse the image area by moving the stencil and the bed in unison relative to a stationary squeegee. This same variation is used when cylindrical objects such as bottles or glasses are stencil printed. On the whole, though, adaption or modification of the individual characteristics of the basic sequence does not change the function of the sequence. The ink is still forced through a frame-supported stencil by a squeegee to produce the desired impression.
A brief description of some basic characteristics of screens and the screen/stencil printing process is helpful in understanding the wide application of this technology. First, the screens that support the stencils are woven in a range of fineness from as coarse as 16 threads per inch, with a thread thickness of 0.0138 inches to as fine as 1635 threads per inch with a thread thickness of 0.0008 inches. Since the emulsion coating completely encapsulates the screen and the ink is deposited relative to the emulsion thickness, the thickness of the impression from the stencil is equal to a calculable amount based on the thread thickness and the ink film thickness after drying. In applications such as electronic circuit printing or plating and solder resist printing, this ability to control film thickness for functional purposes is a practical and economic use of stencil printing.
The nature of this printing technique also contributes to its wide application. In its most basic form, screen/stencil printing requires nothing more than a screen supported stencil and squeegee to produce an impression. No mechanism to actuate pressure is needed. For this reason, stencils may print virtually any size without requiring a machine of corresponding dimension. It is only necessary to be able to sufficiently contact the object to be printed. The impression is made by fluid pressure of the ink and the surface attraction of the material under the stencil. Thus screen/stencil printing is versatile enough to print on large, solid objects and minute, delicate objects with virtually the same pressure.
Together, these characteristics make "silk screen" printing a unique printing process fulfilling product demands that establish it as an essential technology in contemporary manufacturing. Nevertheless, the existing process suffers from significant limitations; the most notable of which is image distortion.
The inventor has identified two primary sources of printed image distortion occurring in the existing screen/stencil printing process. The first arises from the use of a squeegee transversing a stencil to apply an image to a substrate. When this instrument is drawn across the stencil-supporting screen fabric an amount of friction-produced stretch and accompanying image elongation is inevitable. Further, the sqeegee produces undesirable vibration, and static electricity. The latter can attract dust and other particles in the air producing a glitch in the printed image.
The second principal source of distortion arises because the stencil must be supported a slight distance above the substrate in the printing stage to permit the necessary "peel" of the stencil behind the squeegee that assures good edge definition in the printed image; while, in the photoexposure process, the screen frame is held in planar contact with the glass during the vacuum hold. Since the image is exposed at one level and deflected to another for printing, an additional degree of distortion is also inevitable.
Attempts have been made in the past to eliminate the squeegee from the stencil printing process. See for example, U.S. Pat. Nos. 3,172,358, 3,221,648 and 3,221,649 to F. Weiss. These patented devices employ a vacuum induced through the printing bed to suck ink through the stencil and onto a substrate. This process, however, appears to require a porous substrate which must be prewet with solvent. These requirements coupled with the convex printing bed and the absence of any mechanism for quickly releasing the stencil from the substrate, makes this apparatus unsuitable for fine, close tolerance, printing applications. Note further that there is no recognition in these patents of the second principal source of image distortion discussed above and that the patented structure is inherently incapable of addressing the second problem.
U.S. Pat. No. 3,964,385 is directed to a "Unitary Device and Method For Screen Manufacture and Printing" but fails to recognize and redress the image distortion problems discussed above.
When close tolerances and minimum distortion are required in the screen printing process, for example, in electronics applications, a number of cumbersome procedures have been employed to overcome the distortion problems. These have entailed much time consuming analysis and art modification. A need thus persists for a stencil printing process and apparatus which can effectively overcome the above described drawbacks of the existing technology.